1. Field of the Invention
The invention relates to a method for evaporating given components from initial multiple-substance mixtures and multiple-substance systems at subatmospheric pressure in a vacuum chamber in which individual portions of the multiple-substance mixture or multiple-substance system are arranged in ring crucibles at a plurality of levels, from which vapors of the lower-boiling component in each case are withdrawn through at least one vapor exhaust duct, the topmost ring crucible being covered except for the vapor exhaust duct.
2. Description of the Related Technology
The purification of pasty, flowing, fluid or liquefied multiple-substance mixtures or systems by heating in a vacuum and evaporating relatively lower-boiling components becomes more difficult when the boiling points are close together at a given pressure (specific vapor pressure curves of the single components). The difficulties increase further when very low residual contents of the components to be evaporated are required, as for example when trace elements and/or trace compounds interfere with the application for which the purified material is intended. These applications especially appear in the fields of electronics (semiconductor technology) and optics (lens materials), the term, xe2x80x9cmultiple-substance mixtures and systems,xe2x80x9d being understood to mean alloys, glasses, sludges, etc., that is to say, multiple-substance mixtures containing solid-and-solid, solid-and-fluid and fluid-and-fluid phases.
Such multiple-substance mixtures and systems may also contain water, hydrocarbons, mercury, sulfur, zinc, cadmium, sodium, lithium, calcium, antimony, lead, manganese, selenium and tellurium.
To obtain short diffusion distances to the phase boundary surfaces (e.g., fluid-to-vapor) in the wholly or partially fluid multiple-substance mixtures and systems, it is desirable to divide the charge into single portions at several levels with a shallow depth of fill and a favorable ratio of surface area to volume, as is described in connection with resistance heating in DE 31 44 284 C2, which deals with a different subject, namely the embrittlement of hard metals in machining technology. In a stacked arrangement, however, temperature equalization is lost due to convection flows and/or inductive stirring between the individual portions. The consequence is considerable axial and radial temperature differences between the single portions or levels in the stack and thus different amounts of residual impurities in the individual portions, even in the radial direction.
When the entire charge is held in a single crucible, convection flows and/or inductive stirring can produce uniformity, to a certain extent, of temperature and composition of the charge axially and radially. However, convection flows presume temperature differences, and in both cases certain torus-like flow patterns develop, and individual elements of the volume of the liquid are momentarily in the vicinity of the phase boundary (evaporative surface). On average, with respect to the amount of the charge, considerably long diffusion paths are formed, and production suffers. Upon cooling, the melt gradually xe2x80x9cfreezesxe2x80x9d and the solid-to-liquid phase boundary pushes particular impurities ahead of it into the liquid partxe2x80x94the so-called residual meltxe2x80x94and thereby causes gradients in the impurities in the block that finally solidifies. That is a consequence of the action of the effective coefficient of distribution.
keff=Csolid/Cliquid
In this case the removal of the heat of solidification over great distances is troublesome. Moreover, the migration of the phase boundary presumes temperature differences.
Now, it is common practice to perform such processes in vacuum furnaces which have a cylindrical wall made of a material permeable to magnetic fields, such as quartz or plastic (glass fiber-reinforced) and an induction coil arranged outside of the furnace wall.
Now, it is not possible in all cases to hit upon the temperature selection for fractional distillation on the basis of isolated considerations of the specific equilibrium vapor pressure curves of the substances, since such evaporations take place under xe2x80x9cnon-equilibrium conditions.xe2x80x9d Evaporation under such conditions represents the typical, real case with which one has to deal in practice. The evaporation process as a whole is considered to be a transporting of components out of the bulk into the vapor phase, or into the vacuum in some cases, in specific steps, and both the whole process and each individual step can be characterized by specific kinetic parameters. The first step is the transport of the particular component through the diffusion boundary layer, and this transport is proportional to the concentration gradient in this boundary layer. The second step is the free evaporation of the particular component at the surface, and this evaporation is proportional to the concentration of the component at the surface.
At the same time, however, the individual components exercise different interactions with one another, which shift the values from the equilibrium vapor pressure curves against one another. In the case of alloys, this takes place, for example, by the formation of intermetallic phases. These interactions must and can be determined experimentally.
See in this connection the book, xe2x80x9cMetallurgische Thermochemie,xe2x80x9d by Kubaschewski and Evans, 1959, VEB Verlag Technik, Berlin, pages 53 to 55, in which the authors deal with the concept of xe2x80x9cactivityxe2x80x9d of the substances and components in a solution and refer to bonding and attraction forces between the molecules which have to be overcome, and to repulsive forces which can be exploited.
The individual problems and their possible solutions and effects are therefore diametrically opposed.
The invention is therefore addressed to the problem of offering a method and an apparatus by which short diffusion path lengths and great uniformities in the temperature configuration in the radial and axial directions are possible.
The solution of the stated problem is therefore accomplished according to the invention in the method described above by the fact that:
a) ring crucibles of a material which acts as a susceptor towards alternating electromagnetic fields,
b) under the lowest ring crucible and over the topmost ring crucible a heat body is disposed, made of a material which likewise acts as a susceptor towards alternating electromagnetic fields,
c) the ring crucibles and the heat bodies are contained in at least one induction coil and are heated by the inductive coupling such that the lowest ring crucible and the top ring crucible are additionally heated by the heat bodies, and
d) the ring crucibles are heated with timing such that at least one of the given components of the multiple-substance mixture or system is obtained in the greatest possible purity.
Thus the advantages are achieved that short diffusion path lengths and great uniformities in the temperature configuration in the radial and axial direction are possible, and especially that the axial and radial temperature differences between the individual portions or stack levels are extremely small, so that the residual impurity contents in the individual portions can be minimized, even in the radial direction.
The collection of the high-purity component can be accomplished in two alternative ways, namely,
1. by holding back the given component or components in the ring crucibles, or
2. by capturing the given component or components in a condenser independent of the other components. In this case a condenser is used that is not already xe2x80x9coccupiedxe2x80x9d by other substances.
The invention also relates to an apparatus for evaporating given components from initial multiple-substance mixture and system at subatmospheric pressure with a vacuum chamber and a furnace wall in which ring crucibles are arranged at various levels, each crucible having a vapor exhaust passage, while the topmost ring crucible is closed except for the vapor exhaust passage.
The solution of the stated problem is achieved by the invention, therefore, in the apparatus described above in that
a) the ring crucibles consist of a material which acts as a susceptor of alternating electromagnetic fields;
b) under the lowermost ring crucible and above the topmost ring crucible there is disposed at least one heat body made of a material which likewise acts as a susceptor of alternating electromagnetic fields;
c) the furnace wall, the ring crucibles and the heat bodies are surrounded by at least one induction coil, and
d) the lowermost ring crucible and the topmost ring crucible are in thermal exchange with the heat bodies.
It is furthermore especially advantageous, as a result of further developments of the subject of the invention if, either individually or in combination:
the heat bodies are in the form of hollow bodies and if their inside surfaces are in visual contact with a cover of the topmost ring crucible and with the bottom of the lowermost ring crucible,
the height of the bottom heater, the height of the crucible stack and the height of the upper heat body are in a ratio of 1:(0.8 to 2.0):1,
the height of the crucible stack is in a ratio to its outside diameter of 1:(0.2 to 1.5),
the heat bodies are in the form of hollow cylinders,
the top heat body is closed off at the top by a thermal insulating device,
the bottom heat body rests on a ring plate made of a susceptor material, which in turn is disposed on a hollow cylindrical thermal insulating device,
the induction coil consists of a spiral metal band and its cross section in the middle of the axial length of the induction coil is greater than at the ends of the induction coil,
the cross section of the induction coil is a rectangle, if the gaps between the windings on the entire length of the metal band are at least substantially equal, and if the longest axis of the rectangle is axis-parallel and in the center of the coil has a maximum dimension and at the ends of the coil has a minimum dimension,
the induction coil consists of a hollow profile,
the ratios of the greatest to the smallest dimension amount to between 1.1 and 4.0.
the vapor exhaust duct runs from the bottom of the lowermost ring crucible through the ring plate and the thermal insulating device to a condenser, and/or
the furnace wall consists of a quartz tube which is lined on its interior with a thermal insulation of a soft felted material.